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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

215

250-300 c.c. and electrolysed at the ordinary temperature with a current
of 0-5-1 amp., using Winkler electrodes ; duration, 12-15 hours.

When the deposition is complete (see p. 213), the electrolytic beaker
is replaced, without interrupting the current, by another small beaker
containing water faintly acidified with sulphuric acid, the cathode being
detached after some time and washed first with water, then with alcohol,
and lastly with ether ; it is then dried at 70° and weighed. The increased
weight represents copper and any silver present; the latter metal is deter-
mined as in 9 (below).1

If the copper deposited is not of a good, brilliant colour, but appears
brownish and spotted (presence of arsenic or bis IT. nth), it is redissolved in
a mixture of 5-7 c.c. of cone, sulphuric acid and 15-20 c.c. of nitric acid
(D 1-33), diluted to 250-300 c.c. and again electrolysed with addition of a
little powdered lead sulphate (0-4 gram) and ferric sulphate (0-5 gram)
to prevent traces of arsenic and bismuth from being deposited with the
copper.

The residual liquid free from copper, together with the wash water,
serves for the determination of antimony (see 5).

2. Determination of the Bismuth.2—(a] ELECTROLYTICALLY. 10
grams of the sample are dissolved in 50 c.c. of nitric acid (D 1-33), 10 c.c.
of sulphuric acid being added when the action is complete and the solution
evaporated to dryness. The residue is taken up in 200 c.c. of water con-
taining 5 c.c. of sulphuric acid, the liquid being heated to boiling and the
boiling liquid treated with 10 c.c. of phosphoric acid (D 1-71). When cold
it is mixed with 30 c.c. of alcohol and, after 12 hours, filtered by decanta-
tion. The precipitate, which contains all the lead and bismuth of the
sample, is washed first with a solution containing by volume about i%
of sulphuric acid, 5% of phosphoric acid and 15% of alcohol, and then with
a dilute solution of ammonium sulphide and potassium cyanide 3 to remove
the last traces of copper, arsenic, antimony, etc. The precipitate is then
dissolved in the hot in nitric acid diluted with an equal volume of water,
the liquid filtered by decantation and the residue treated with aqua regia
diluted with an equal volume of water, filtered and washed with boiling
water. The solution is evaporated with 12 c.c. of sulphuric acid until
copious white fumes of sulphuric acid appear and, when cold, is treated

1  In presence of large quantities of silver, the electrolysis should be started at 30°
•with a current of o-i amp. to deposit all the silver first.    After some hours the current
intensity is raised to 0-5-1 amp. and the electrolysis continued at the ordinary tem-
perature until all the copper is deposited.

2  Detection of bismuth in copper (Abel and Field).—About 6 grams of the sample are
dissolved in nitric acid, treated with about 0-3 gram of lead nitrate dissolved in a little
water, with ammonia until the reaction is alkaline and with a little ammonium carbonate.
After standing a little while, the liquid is filtered, the precipitate washed with ammonia-
cal water and dissolved in hot acetic acid, and sufficient potassium iodide, dissolved in a
little water, added to dissolve in the hot the precipitate at first formed.    As it cools, the
solution deposits lead iodide crystals which, instead of pale yellow, are orange or red
in presence of bismuth.    By this method, 0-02 m, grm. of bismuth is detectable.    A
blank experiment with pure lead nitrate should be made and the colour of the lead
iodide compared with that obtained in presence of the copper.

3  100 c.c. of this solution should contain 5 grams of potassium cyanide and 5 c.c.
of ammonium sulphide prepared by saturation of 10 % ammonia with hydrogen sulphide.               -JJ.-IO               3-06             3-70